Upon loss of existing tissue integrity, wound closure is critical for maintenance of the protective epidermal barrier that shields the host from environmental insult and prevents water loss. An essential, initial response during wound repair is the influx of circulating leukocytes that cleanse the wound and also provide activators and growth factors to stimulate fibrogenesis and neovascularization. Members of the matrix metalloproteinase (MMP) family, such as MMP3, are potential key regulators of leukocyte infiltration. MMP3 is upregulated in the epidermis during processes undergoing tissue remodeling. Mice which are null for MMP3 have delayed closure of excisional wounds and demonstrate reduced infiltration of leukocytes in a number of model systems. Furthermore, we find that MMP3-/- animals have an increased sensitivity to SCC progression that is associated with a reduction in neutrophil and macrophage infiltration evident throughout all stages of tumor progression. Based upon these observations, we propose that MMP3 regulates a protective host defense response during wound repair that is reflective of a protective, antitumorigenic response during SCC. A number of molecules known to affect immune cell function can be directly targeted for hydrolysis and release by MMP3 or matrix proteinases which are activated by MMP3. We hypothesize that MMP3, in combination with other matrix proteinases, directly enhances leukocyte extravasation that subsequently influences the wound repair process. Leukocyte infiltration into sites of inflammation requires the coordinate regulation of multiple steps including arrest on endothelium, migration through the endothelial barrier and directed migration through connective tissue towards the site of inflammation. MMP3 is expressed by a variety of cell types including keratinocytes, fibroblasts, endothelial cells and monocytes and can activate other matrix proteinase classes; thus, MMP3 and other MMPs may modulate multiple key steps required for optimal leukocyte extravasation. The goal of this project is to elucidate the functional role of MMP3 in leukocyte extravasation during wound healing. Our specific aims will test our hypothesis as follows: Specific Aim 1: To identity the cellular matrix proteinase contributions of leukocytes, fibroblasts and keratinocytes to MMP3 dependent leukocyte infiltration into the wound environment using in vivo mouse models that are null for or have targeted overexpression of individual MMP family members. Specific Aim 2. To test the hypothesis that MMP3 activity regulates activation of subsequent MMP family members in wounded epidermis. Specific Aim 3: To identity the cellular mechanisms regulated during MMP3 and matrix protease dependent transendothelial migration in vitro. These studies will provide novel insights into the role of MMP3 and other matrix proteinases during leukocyte transendothelial migration and provide an in vivo model system for testing MMP inhibitors in a clinical setting.